Lens system with fill-in lens

ABSTRACT

A lens system having a main photo-taking lens with a fill-in lens detachably set in between the main photo-taking lens and the image plane to change the focal length of the entire system. This fill-in lens has, from front to rear, a positive 1st lens element, a negative 2nd lens element and a positive 3rd lens element. The glass of the positive 1st lens element and the lens form and glass of the positive 3rd lens element are properly chosen to minimize the bulk and size, while still achieving improvements of the image quality.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to optical systems of which the focal length ischanged by attaching or detaching a fill-in lens in between the mainphoto-taking lens and the image plane and more particularly to compactand highly correct fill-in lenses.

2. Description of the Prior Art

It is generally known in the art to elongate or shorten the focal lengthof a photographic objective by attaching a converter lens in front or inrear of the photographic objective in such a manner as to maintain afixed focal plane. In this case, the positioning on the object side ofthe photographic objective, or the front converter lens type requires abulky converter lens and particularly a front lens element of undulylarge diameter, and is difficult to reduce this. Unlike this, the rearconverter lens type which arranges the converter lens on the image sideof the photographic objective is more advantageous in shortening thelens diameter than the above-mentioned front converter type.

A lens system which may be considered to have taken this kind ofadvantage is exemplified in Japanese Laid Open Patent No. Sho 54-97423where use is made of a 6-component converter lens, and, in order tochange the focal length of the main photo-taking lens, it is requiredthat the main photo-taking lens is moved forward relative to the camerabody, thus elongating the void which later accommodates the converterlens.

On the other hand, the present applicant has proposed in U.S. patentapplication Ser. No. 219,977 that the number of lens elementsconstituting the converter lens is reduced so that after the converterlens has been put in between the main lens and the camera body, theaccommodating space is shortened in the longitudinal length, therebygiving an additional advantage in that a further minimization of thebulk and size of the camera body is facilitated.

Another aspect of the camera having this kind of converter lens (fill-inlens) incorporating provision is that the diaphragm mechanism and theshutter mechanism are positioned in a space between the mainphoto-taking lens and the fill-in lens. Therefore, the wider theseparation between both lenses, the more advantageously these mechanismsare assembled, and the interference of their lens mountings is avoided.But, this gives rise to a problem wherein as the distance from the mainphoto-taking lens to the film plane increases, the bulk and size of thecomplete camera is increased. Then, if a compromise between therequirements of widening the axial separation between both lenses and ofproviding for a shortening of the physical length of the entire lenssystem is to be made, the negative refractive power of the fill-in lenshas to be strengthened which causes the Petzval sum to be increased inthe negative sense. Therefore, as the meridional image surface andsagittal image surface incline toward a negative direction, thecurvature of field is increased, and strong outward coma is produced.

As additional examples of the arrangement of the fill-in lens to therear of the main photo-taking lens, reference may be made to U.S. Pat.Nos. 3,741,621 and 4,206,971.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a fill-in lens whichalthough compact, fulfills the requirement of providing a high standardof aberration correction. Accordingly, the use of the fill-in lens in acamera assures that a space sufficiently large to accommodate mechanismssuch as the diaphragm is left aside and gives rise to the possibility ofachieving stability of the aberrations during the changing of the focallength. The construction and arrangement of the lens of the invention issuch that the fill-in lens is attached at a point of position betweenthe main photo-taking lens and the image plane in a releasable manner,and has, from front to rear, a positive 1st lens element, a negative 2ndlens element, and positive 3rd lens element. And, the refractive indexand Abbe number of the glass of which the positive 1st lens element ismade up, the figuring of the positive 3rd lens, and the refractive indexand Abbe number of the glass of which the positive 3rd lens element ismade up are so appropriately specified to accomplish the abovedescribedobject.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a lens block diagram of an example of a main photo-takinglens.

FIGS. 2a, 2b and 2c are graphic representations of the variousaberrations of the lens of FIG. 1 with object at infinity.

FIGS. 3, 5, 7, 9, 11, 13, 15, and 17 are lens block diagrams of 1st to8th embodiments of fill-in lenses according to the present inventionattached to the main photo-taking lens of FIG. 1 respectively.

FIGS. 4a to 4c, 6a to 6c, 8a to 8c, 10a to 10c, 12a to 12c, 14a to 14c,16a to 16c and 18a to 18c are graphic representations of the variousaberrations of the 1st to 8th lens systems respectively with object atinfinity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The fill-in lens of the present invention may be used in a space betweenthe main photo-taking lens and the image plane, is releasably mountedtherein, and has a positive 1st lens element, a negative 2nd lenselement and a positive 3rd lens element in this order from the front,being characterized by the following conditions:

(1) 0.3<r₅ /f<0.6

(2) 0.02<d₅ /f<0.08

(3) 1.5<n₁ <1.7

(4) 30.0<ν₁ <50.0

(5) 1.45<n₃ <1.75

(6) 45.0<ν₃ <85

where n₁ and ν₁ are the refractive index for the spectral d line and theAbbe number of the glass of the 1st lens element respectively; r₅ is theradius of curvature of the front surface of the 3rd lens element, and d₅is the axial thickness of the same; n₃ and ν₃ are the refractive indexfor the spectral d line and the Abbe number of the glass of the 3rd lenselement; and f is the overall focal length of the main photo-taking lensand the fill-in lens.

Next, the significance of each of the above-described various conditionswill be explained.

Condition (1) is concerned with the increase of astigmatic differenceand the correction of outward coma which give rise to a problem when theseparation between the main photo-taking lens and the fill-in lens iswidened and at the same time the combined physical length of the mainphoto-taking lens and the fill-in lens is shortened. That is, when thevalue of r₅ /f exceeds the lower limit, though it is advantageous incorrecting the coma, the refractive power of this or front lens surfacebecomes too strong for avoidance of under-correction of the meridionalimage surface with the result that the astigmatic difference increases.When the upper limit is violated, it is a minimization of the Petzvalsum that can be made possible, and therefore what is well corrected isastigmatism. But, since the refractive power of this lens surface is tooweak, coma is not sufficiently corrected and outward coma is left.Therefore, in order to achieve good correction of astigmatism and coma,condition (1) is set forth.

Condition (2) similarly with condition (1) is concerned in particular,with correction of astigmatism and coma. That is, when the value of d₅/f exceeds the lower limit, the refractive power of the 3rd lens elementis too weak for avoidance of under-correction of the meridional imagesurface and sufficient correction of astigmatism cannot be effected.When the upper limit is exceeded, the refractive power of the 3rd lenselement becomes strong with an advantage at correction of astigmatism.In this case, however, strong outward coma is produced. Therefore,condition (2) is of importance to facilitating good correction ofastigmatism and coma.

Condition (3) is concerned in particular, with correction of coma. Thatis, when the value of n₁ exceeds the lower limit, strong inward coma isproduced. When the upper limit is exceeded, strong outward coma isproduced. In order to achieve a balanced correction of coma, condition(3) is necessary.

Condition (4) is concerned with correction of chromatic aberrations.That is, when the value of ν₁ exceeds the lower limit, longitudinal andlateral chromatic aberrations are both positive and increased. When theupper limit is exceeded, longitudinal and lateral chromatic aberrationsare both increased in the negative sense. Therefore, condition (4) isnecessary to correction of chromatic aberrations.

Condition (5) has is particularly concerned with correction ofdistortion. That is, it is intended that the strong positive distortionproduced from the main photo-taking lens and the positive 1st andnegative 2nd lens elements in the fill-in lens is balanced out with thenegative distortion produced in the 3rd lens element of the fill-inlens. When the value of the refractive index n₃ of the 3rd lens elementin the fill-in lens exceeds the upper limit of the inequalities (5),though good correction of distortion can be achieved, the Petzval sum ispositive and increased, calling for under-correction of the meridionalimage surface. Therefore, an increase in the astigmatic differenceresults. Therefore, condition (5) is necessary to the achievement ofgood balanced correction of distortion and astigmatism.

Condition (6) is concerned with correction of chromatic aberrations.That is, when the value of ν₃ exceeds the lower limit, the longitudinaland lateral chromatic aberrations are both increased in the negativesense. Therefore, condition (6) is necessary to the achievement of goodcorrection of chromatic aberrations.

It is noted here that the lower limit of the inequalities (5) and theupper limit of the inequalities (6) depend upon the presently existingglass materials. Their values may be specified for the time being as1.45 for the lower limit in condition (5), and as 85 for the upper limitin condition (6).

In the specific embodiments, the positive 1st lens element of thefill-in lens turns its strong convex side (the radius of curvature issmaller in the absolute value than that of curvature of the oppositeside) towards the image plane, the negative 2nd lens element in thebi-concave form turns its stronger concave side towards the object, andthe positive 3rd lens element turns its strong convex side towards theobject. The fact that these figurings advantageously work on theaberration correction is pointed out in the before-mentioned preceedingpatent application.

An example of a specific main photo-taking lens to which the fill-inlens of the invention is to be attached is first given in the followingtable for the radii of curvature, R_(i), of the lens surfaces, the axialthicknesses and air separations, D_(i), of the lens elements and betweenthe lens elements, the refractive index, N_(i), for the spectral d lineof the glass material of the i-th lens element, and the Abbe number,ν_(i), of that glass material.

    ______________________________________                                        Focal Length f = 100 F-number = 1:2.8 Image Angle 2ω = 59.3°     Back Focus b.f. = 77.152                                                      R.sub.1 = 34.595                                                                         D.sub.1 = 10.717                                                                          N.sub.1 = 1.77250                                                                         V.sub.1 = 49.6                             R.sub.2 = 109.667                                                                        D.sub.2 = 3.089                                                    R.sub.3 = -158.806                                                                       D.sub.3 = 5.266                                                                           N.sub.2 = 1.72151                                                                         V.sub.2 = 29.2                             R.sub.4 = 32.754                                                                         D.sub.4 = 3.555                                                    R.sub.5 = 260.171                                                                        D.sub.5 = 2.054                                                                           N.sub.3 = 1.58144                                                                         V.sub.3 = 40.7                             R.sub.6 = 38.414                                                                         D.sub.6 = 8.584                                                                           N.sub.4 = 1.80610                                                                         V.sub.4 = 40.9                             R.sub.7 = -77.985                                                                        D.sub.7 = 4.213                                                    R.sub.8 = 0. (Stop)                                                           ______________________________________                                    

Next, specific embodiments of the lens system comprising theabove-described main photo-taking lens and a fill-in lens are givenbelow.

It should be pointed out in connection with the following embodimentsthat, when the fill-in lens is attached, the main phototaking lens isshifted forwards to bring the position of the resultant focal plane intocoincidence with that of the focal plane for the main photo-taking lensalone.

    ______________________________________                                        1st Embodiment (FIGS. 3 and 4)                                                f = 154.287                                                                              F-No. 1:4.4                                                                              2ω = 40.5°                                                                   b.f. = 89.753                               ______________________________________                                        R.sub.1 = 34.595                                                                         D.sub.1 = 10.717                                                                         N.sub.1 = 1.77250                                                                         V.sub.1 = 49.6                              R.sub.2 = 109.667                                                                        D.sub.2 = 3.089                                                    R.sub.3 = -158.806                                                                       D.sub.3 = 5.266                                                                          N.sub.2 = 1.72151                                                                         V.sub.2 = 29.2                              R.sub.4 = 32.754                                                                         D.sub.4 = 3.555                                                    R.sub.5 = 260.171                                                                        D.sub.5 = 2.054                                                                          N.sub.3 = 1.58144                                                                         V.sub.3 = 40.7                              R.sub.6 = 38.414                                                                         D.sub.6 = 8.584                                                                          N.sub.4 = 1.80610                                                                         V.sub.4 = 40.9                              R.sub.7 = -77.985                                                                        D.sub.7 = 4.213                                                    R.sub.8 = 0. (Stop)                                                                      D.sub.8 = 4.477                                                    r.sub.1 = 0.                                                                             d.sub.1 = 5.740                                                                          n.sub.1 = 1.59270                                                                         ν.sub.1 = 35.3                           r.sub.2 = -36.679                                                                        d.sub.2 = 2.265                                                    r.sub.3 = -33.601                                                                        d.sub.3 = 2.159                                                                          n.sub.2 = 1.77250                                                                         ν.sub.2 = 49.6                           r.sub.4 = 93.497                                                                         d.sub.4 = 0.263                                                    r.sub.5 = 67.147                                                                         d.sub.5 = 6.846                                                                          n.sub.3 = 1.49831                                                                         ν.sub.3 = 65.0                           r.sub.6 = -633.479                                                            ______________________________________                                    

    __________________________________________________________________________    3rd Order Aberration Coefficients (with Inclusion of Main Lens)               No. L    T    SA   CM   AS   PT   DS                                          __________________________________________________________________________    R.sub.1                                                                           0.04889                                                                            -0.00133                                                                           21.8115                                                                            -0.5940                                                                            0.0162                                                                             1.9437                                                                             -0.0534                                     R.sub.2                                                                           0.01222                                                                            -0.01348                                                                           2.0220                                                                             -2.2315                                                                            2.4625                                                                             -0.6131                                                                            -2.0409                                     R.sub.3                                                                           -0.05040                                                                           0.02772                                                                            -16.4071                                                                           9.0239                                                                             -4.9632                                                                            -0.4072                                                                            2.9537                                      R.sub.4                                                                           -0.06356                                                                           -0.00946                                                                           -20.0621                                                                           -2.9852                                                                            -0.4442                                                                            -1.9742                                                                            -0.3599                                     R.sub.5                                                                           0.01075                                                                            0.01037                                                                            0.5828                                                                             0.5621                                                                             0.5422                                                                             0.2180                                                                             0.7332                                      R.sub.6                                                                           0.01018                                                                            0.00136                                                                            6.2538                                                                             0.8323                                                                             0.1108                                                                             0.3159                                                                             0.0568                                      R.sub.7                                                                           0.03387                                                                            -0.01545                                                                           11.0172                                                                            -5.0240                                                                            2.2910                                                                             0.8830                                                                             -1.4474                                     R.sub.8                                                                       r.sub.1                                                                           -0.01483                                                                           0.01289                                                                            -1.5588                                                                            1.3554                                                                             -1.1786                                                                            0.0  1.0248                                      r.sub.2                                                                           0.05466                                                                            -0.00794                                                                           61.9026                                                                            -8.9918                                                                            1.3061                                                                             1.5653                                                                             -0.4171                                     r.sub.3                                                                           -0.04086                                                                           0.00518                                                                            -64.7666                                                                           8.2105                                                                             -1.0408                                                                            -2.0012                                                                            0.3856                                      r.sub.4                                                                           -0.00517                                                                           -0.01180                                                                           -0.0326                                                                            -0.0745                                                                            -0.1699                                                                            -0.7192                                                                            -2.0280                                     r.sub.5                                                                           0.00446                                                                            0.00700                                                                            0.1515                                                                             0.2381                                                                             0.3744                                                                             0.7642                                                                             1.7900                                      r.sub.6                                                                           0.00416                                                                            -0.00522                                                                           0.4443                                                                             -0.5577                                                                            0.6999                                                                             0.0810                                                                             -0.9801                                     Total                                                                             0.00435                                                                            -0.00015                                                                           1.3584                                                                             -0.2363                                                                            0.0064                                                                             0.0562                                                                             -0.3826                                     __________________________________________________________________________     L: Lateral Chromatic Aberration Coefficient                                   T: Longitudinal Chromatic Aberration Coefficient                              SA: Spherical Aberration Coefficient                                          CM: Coma Coefficient                                                          As: Astigmatism Coefficient                                                   PT: Petzval Sum                                                               DS: Distortion Coefficient                                               

In the following tables, the numerical data concerning with the mainphoto-taking lens are omitted. However, the focal length f, F-number,image angle and back focus are described with their values taken whenthe fill-in lens in the corresponding embodiment is attached to theabove-described main photo-taking lens.

    ______________________________________                                        2nd Embodiment (FIGS. 5 and 6)                                                f = 152.536                                                                              F-No. 1:4.4                                                                              2ω = 40.9°                                                                   b.f. = 88.314                               ______________________________________                                        R.sub.8 = 0. (Stop)                                                                      D.sub.8 = 4.474                                                    r.sub.1 = 0.                                                                             d.sub.1 = 5.040                                                                          n.sub.1 = 1.59270                                                                         ν.sub.1 = 35.3                           r.sub.2 = -37.732                                                                        d.sub.2 = 2.286                                                    r.sub.3 = -34.630                                                                        d.sub.3 = 4.189                                                                          n.sub.2 = 1.77250                                                                         ν.sub.2 = 49.6                           r.sub.4 = 61.126                                                                         d.sub.4 = 0.263                                                    r.sub.5 = 55.485                                                                         d.sub.5 = 6.981                                                                          n.sub.3 = 1.7200                                                                          ν.sub.3 = 50.2                           r.sub.6 = 1468.555                                                            ______________________________________                                    

    ______________________________________                                        3rd Embodiment (FIGS. 7 and 8)                                                f = 155.362                                                                              F-No. 1:4.4                                                                              2ω = 40.3°                                                                   b.f. = 89.707                               ______________________________________                                        R.sub.8 = 0. (Stop)                                                                      D.sub.8 = 4.771                                                    r.sub.1 = 0.                                                                             d.sub.1 = 5.809                                                                          n.sub.1 = 1.59270                                                                         ν.sub.1 = 35.3                           r.sub.2 = -36.844                                                                        d.sub.2 = 2.304                                                    r.sub.3 = -33.729                                                                        d.sub.3 = 2.330                                                                          n.sub.2 = 1.77250                                                                         ν.sub.2 = 49.6                           r.sub.4 = 96.909                                                                         d.sub.4 = 0.263                                                    r.sub.5 = 65.452                                                                         d.sub.5 = 6.567                                                                          n.sub.3 = 1.46450                                                                         ν.sub.3 = 66.0                           r.sub.6 = -693.008                                                            ______________________________________                                    

    ______________________________________                                        4th Embodiment (FIGS. 9 and 10)                                               f = 153.723                                                                              F-No. 1:4.4                                                                              2ω = 40.7°                                                                   b.f. = 89.130                               ______________________________________                                        R.sub.8 = 0. (Stop)                                                                      D.sub.8 = 5.413                                                    r.sub.1 = -2729.781                                                                      d.sub.1 = 5.279                                                                          n.sub.1 = 1.59270                                                                         ν.sub.1 = 35.3                           r.sub.2 = -36.854                                                                        d.sub.2 = 2.318                                                    r.sub.3 = -33.771                                                                        d.sub.3 = 3.351                                                                          n.sub.2 = 1.77250                                                                         ν.sub.2 = 49.6                           r.sub.4 = 93.298                                                                         d.sub.4 = 0.263                                                    r.sub.5 = 67.147                                                                         d.sub.5 = 4.740                                                                          n.sub.3 = 1.49831                                                                         ν.sub.3 = 65.0                           r.sub.6 = -513.954                                                            ______________________________________                                    

    ______________________________________                                        5th Embodiment (FIGS. 11 and 12)                                              f = 152.609                                                                              F-No. 1:4.4                                                                              2ω = 40.9°                                                                   b.f. = 89.070                               ______________________________________                                        R.sub.8 = 0. (Stop)                                                                      D.sub.8 = 4.475                                                    r.sub.1 = 0.                                                                             d.sub.1 = 5.055                                                                          n.sub.1 = 1.59270                                                                         ν.sub.1 = 35.3                           r.sub.2 = -36.773                                                                        d.sub.2 = 2.288                                                    r.sub.3 = -33.662                                                                        d.sub.3 = 2.043                                                                          n.sub.2 = 1.77250                                                                         ν.sub.2 = 49.6                           r.sub.4 = 94.561                                                                         d.sub.4 = 0.263                                                    r.sub.5 = 66.521                                                                         d.sub.5 = 10.533                                                                         n.sub.3 = 1.49831                                                                         ν.sub.3 = 65.0                           r.sub.6 = -631.474                                                            ______________________________________                                    

    __________________________________________________________________________    3rd Order Aberration Coefficients (with Inclusion of Main Lens)               No. L    T    SA   CM   AS   PT   DS                                          __________________________________________________________________________    R.sub.1                                                                           0.04836                                                                            -0.00133                                                                           21.1065                                                                            -0.5812                                                                            0.0160                                                                             1.9225                                                                             -0.0534                                     R.sub.2                                                                           0.01208                                                                            -0.01348                                                                           1.9567                                                                             -2.1831                                                                            2.4357                                                                             -0.6065                                                                            <2.0409                                     R.sub.3                                                                           -0.04985                                                                           0.02772                                                                            -15.8768                                                                           8.8284                                                                             -4.9091                                                                            -0.4028                                                                            2.9537                                      R.sub.4                                                                           -0.06287                                                                           -0.00946                                                                           -19.4136                                                                           -2.9206                                                                            -0.4394                                                                            -1.9527                                                                            -0.3599                                     R.sub.5                                                                           0.01063                                                                            0.01037                                                                            0.5639                                                                             0.5499                                                                             0.5363                                                                             0.2157                                                                             0.7332                                      R.sub.6                                                                           0.01007                                                                            0.00136                                                                            6.0516                                                                             0.8143                                                                             0.1096                                                                             0.3125                                                                             0.0568                                      R.sub.7                                                                           0.03350                                                                            -0.01545                                                                           10.6611                                                                            -4.9152                                                                            2.2661                                                                             0.8734                                                                             -1.4474                                     R.sub.8                                                                       r.sub.1                                                                           -0.01467                                                                           0.01290                                                                            -1.5084                                                                            1.3261                                                                             -1.1658                                                                            0.0  1.0249                                      r.sub.2                                                                           0.05457                                                                            -0.00823                                                                           60.7966                                                                            -9.1711                                                                            1.3835                                                                             1.5443                                                                             -0.4417                                     r.sub.3                                                                           -0.04079                                                                           0.00539                                                                            -63.6017                                                                           8.4056                                                                             -1.1109                                                                            -1.9758                                                                            0.4079                                      r.sub.4                                                                           -0.00512                                                                           -0.01176                                                                           -0.0308                                                                            -0.0708                                                                            -0.1627                                                                            -0.7034                                                                            -1.9905                                     r.sub.5                                                                           0.00452                                                                            0.00699                                                                            0.1571                                                                             0.2429                                                                             0.3754                                                                             0.7630                                                                             1.7596                                      r.sub.6                                                                           0.00406                                                                            -0.00507                                                                           0.4323                                                                             -0.5394                                                                            0.6730                                                                             0.0804                                                                             -0.9400                                     Total                                                                             0.00451                                                                            -0.00006                                                                           1.2944                                                                             -0.2141                                                                            0.0076                                                                             0.0707                                                                             -0.3376                                     __________________________________________________________________________

    ______________________________________                                        6th Embodiment (FIGS. 13 and 14)                                              f = 152.658                                                                              F-No. 1:4.4                                                                              2ω = 40.9°                                                                   b.f. = 87.849                               ______________________________________                                        R.sub.8 = 0. (Stop)                                                                      D.sub.8 = 4.462                                                    r.sub.1 = 4216.695                                                                       d.sub.1 = 5.096                                                                          n.sub.1 = 1.59270                                                                         ν.sub.1 = 35.3                           r.sub.2 = -36.441                                                                        d.sub.2 = 2.142                                                    r.sub.3 = -33.800                                                                        d.sub.3 = 3.504                                                                          n.sub.2 = 1.78590                                                                         ν.sub.2 = 44.2                           r.sub.4 = 120.274                                                                        d.sub.4 = 0.263                                                    r.sub.5 = 78.621                                                                         d.sub.5 = 6.578                                                                          n.sub.3 = 1.51742                                                                         ν.sub.3 = 52.4                           r.sub.6 = -5642.191                                                           ______________________________________                                    

    __________________________________________________________________________    No. L    T    SA   CM   AS   PT   DS                                          __________________________________________________________________________    R.sub.1                                                                           0.04836                                                                            -0.00133                                                                           21.1065                                                                            -0.5812                                                                            0.0160                                                                             1.9225                                                                             -0.0534                                     R.sub.2                                                                           0.01208                                                                            -0.01348                                                                           1.9567                                                                             -2.1831                                                                            2.4357                                                                             -0.6065                                                                            <2.0409                                     R.sub.3                                                                           -0.04985                                                                           0.02772                                                                            -15.8768                                                                           8.8284                                                                             -4.9091                                                                            -0.4028                                                                            2.9537                                      R.sub.4                                                                           -0.06287                                                                           -0.00946                                                                           -19.4136                                                                           -2.9206                                                                            -0.4394                                                                            -1.9527                                                                            -0.3599                                     R.sub.5                                                                           0.01063                                                                            0.01037                                                                            0.5639                                                                             0.5499                                                                             0.5363                                                                             0.2157                                                                             0.7332                                      R.sub.6                                                                           0.01007                                                                            0.00136                                                                            6.0516                                                                             0.8143                                                                             0.1096                                                                             0.3125                                                                             0.0568                                      R.sub.7                                                                           0.03350                                                                            -0.01545                                                                           10.6611                                                                            -4.9152                                                                            2.2661                                                                             0.8734                                                                             -1.4474                                     R.sub.8                                                                       r.sub.1                                                                           -0.01467                                                                           0.01290                                                                            -1.5084                                                                            1.3261                                                                             -1.1658                                                                            0.0  1.0249                                      r.sub.2                                                                           0.05457                                                                            -0.00823                                                                           60.7966                                                                            -9.1711                                                                            1.3835                                                                             1.5443                                                                             -0.4417                                     r.sub.3                                                                           -0.04079                                                                           0.00539                                                                            -63.6017                                                                           8.4056                                                                             -1.1109                                                                            -1.9758                                                                            0.4079                                      r.sub.4                                                                           -0.00512                                                                           -0.01176                                                                           -0.0308                                                                            -0.0708                                                                            -0.1627                                                                            -0.7034                                                                            -1.9905                                     r.sub.4                                                                           0.00452                                                                            0.00699                                                                            0.1571                                                                             0.2429                                                                             0.3754                                                                             0.7630                                                                             1.7596                                      r.sub.6                                                                           0.00406                                                                            -0.00507                                                                           0.4323                                                                             -0.5394                                                                            0.6730                                                                             0.0804                                                                              -0.9400                                    Total                                                                             0.00451                                                                            -0.00006                                                                           1.2944                                                                             -0.2141                                                                            0.0076                                                                             0.0707                                                                             -0.3376                                     __________________________________________________________________________

    ______________________________________                                        7th Embodiment (FIGS. 15 and 16)                                              f = 152.614                                                                              F-No. 1:4.4                                                                              2ω = 40.9°                                                                   b.f. = 87.646                               ______________________________________                                        R.sub.8 = 0. (Stop)                                                                      D.sub.8 = 4.831                                                    r.sub.1 = 909.937                                                                        d.sub.1 = 4.916                                                                          n.sub.1 = 1.58267                                                                         ν.sub.1 = 46.4                           r.sub.2 = -35.556                                                                        d.sub.2 = 2.141                                                    r.sub.3 = -32.761                                                                        d.sub.3 = 4.206                                                                          n.sub.2 = 1.77250                                                                         ν.sub.2 = 49.6                           r.sub.4 = 98.906                                                                         d.sub.4 = 0.263                                                    r.sub.5 = 71.466                                                                         d.sub.5 = 6.754                                                                          n.sub.3 = 1.49831                                                                         ν.sub.3 = 65.0                           r.sub.6 = -900.379                                                            ______________________________________                                    

    ______________________________________                                        8th Embodiment (FIGS. 17 and 18)                                              f = 152.662                                                                              F-No. 1:4.4                                                                              2ω = 40.9°                                                                   b.f. = 88.430                               ______________________________________                                        R.sub.8 = 0. (Stop)                                                                      D.sub.8 = 5.182                                                    r.sub.1 = -210.971                                                                       d.sub.1 = 5.319                                                                          n.sub.1 = 1.68893                                                                         ν.sub.1 = 31.1                           r.sub.2 = -40.821                                                                        d.sub.2 = 2.952                                                    r.sub.3 = -35.784                                                                        d.sub.3 = 2.098                                                                          n.sub.2 = 1.77250                                                                         ν.sub.2 = 49.6                           r.sub.4 = 90.032                                                                         d.sub.4 = 0.263                                                    r.sub.5 = 60.857                                                                         d.sub.5 = 7.215                                                                          n.sub.3 = 1.49831                                                                         ν.sub.3 = 65.0                           r.sub.6 = -290.297                                                            ______________________________________                                    

What is claimed is:
 1. A lens system having a main photo-taking lens and a fill-in lens lying in between said main photo-taking lens and the image plane and detachable therefrom, and having the following various conditions:said fill-in lens having, from front to rear, a positive 1st lens, a negative 2nd lens and a positive 3rd lens; letting n₁ and ν₁ denote the refractive index and Abbe number of the glass of said positive 1st lens respectively, r₅ the radius of curvature of the front surface of said positive 3rd lens, d₅ the axial thickness of said 3rd lens, n₃ and ν₃ the refractive index and Abbe number of the glass of said 3rd lens respectively, and f the overall focal length of said main photo-taking lens and said fill-in lens attached thereto, the following relationships are fulfilled: (1) 0.3<r₅ /f<0.6 (2) 0.02<d₅ /f<0.08 (3) 1.5 <n₁ <1.7 (4) 30.0<ν₁ <50.0 (5) 1.45<n₃ <1.75 (6) 45.0<ν₃ <85 